Polymer plastics are ubiquitous in our society. They are used for everything from computer cases to paint. They have replaced many more valuable, non-renewable resources such as wood and metal. Unfortunately, plastics are not very biodegradable. This disadvantage is compounded by the fact that many plastic materials are designed to be used once and then disposed of. In addition, the trash bags into which other plastics are disposed in are made of plastic as well. The large amount of plastics being thrown away has become a serious pollution problem.
Polyethylene (PE), polypropylene (PP), polystyrene (PS), polybutylene (PB), poly(styrene-ethylene-butylene-stryrene) (SEBS), polyvinylfluoride (PVF), polyvinylchloride (PVC), poly(ethylene terephthalate) (PET) are some of the more common plastics. They are non-biodegradable and generally synthesized from petroleum products. Our society has come to depend on these materials, despite their environmental disadvantages. In addition, because they are derived from petroleum, they increase our country's dependence on imported oil. It is, therefore, desirable to find an environmentally friendly alternative to these products.
Several alternative materials have been investigated to replace these plastic polymers. Starch is a very common, environmentally friendly, biopolymer. However, pure starch is brittle and rapidly degrades when exposed to water. To overcome these deficiencies, there have been many attempts to combine starch with synthetic polymers. If starch could be used as an additive with polymers such as those listed above, it would not only increase their biodegradability, but would also make them less expensive to synthesize. For example, low density polyethylene (LDPE) costs about 50¢ per pound as opposed to starch which costs about 15¢ per pound.
Mixtures of starch and polymers have, unfortunately, resulted in materials leaving poor physical qualities. Starch generally does not mix well with the polymers listed above. In an attempt to improve these mixtures, some researchers have used high amylose starch. Although high amylose starch slightly improves the physical properties of the mixtures, it is relatively expensive and reduces or eliminates the cost benefits of these mixtures. Gelatinized starch has also been used in an attempt to improve these mixtures. However, gelatinized starch as well as high amylose starch requires the addition of a plasticizer. Gelatinized starch results from processing raw starch. This increases its cost and, therefore, makes it less economical than raw starch.
The addition of a plasticizer poses another disadvantage. As with requiring processing of the starch, this increases the cost. In addition, plasticizers such as glycerol increase the water absorbency of the polymer and starch mixture. This has a deleterious effect on the mixture's physical properties, making them impractical.
Raw, granular starch comes from a variety of sources, including corn, wheat, rice and potatoes. Wheat and rice granular starch tends to be the least expensive.
U.S. Pat. No. 6,218,532 discusses a method of synthesizing materials from amylose starch derivatives. The starch is chemically modified to form starch ethers, esters, and carbamates. It does not disclose cross-linking of starch to polyethylene.
U.S. Pat. No. 6,235,815 discloses mixing of starch with a hydrophobic biodegradable polymer, including polyethylene oxide. It does not include a compatibilizer to covalently attach polyethylene to granular starch.
U.S. Pat. No. 5,719,214 discloses starch polymer having polyester grafted onto it. It does not disclose polyethylene having granular starch grafted onto it.
U.S. Pat. No. 5,569,692 describes a starch-based composition for the production of articles of biodegradable plastics material. The starch is heated with a destructuring agent in order to destructure the starch. It is then mixed with a polymer, preferably polyvinyl alcohol or ethylene vinyl alcohol. This patent does not disclose covalent grafting of granular starch onto polyethylene.
U.S. Pat. No. 5,461,094 discloses the formation of a biodegradable material by chemically bonding starch with polyethylene. However, the method disclosed in this patent is significantly more complex than the present invention. It requires the use of a radical initiator and acid catalytic co-monomer and auto oxidizing agent and a plasticizer. It does not disclose the simple covalent attachment of granular starch to polyethylene.
It is therefore desirable to develop an inexpensive method for synthesizing a starch/polymer mixture in which inexpensive, unprocessed starch of any variety may be used.